Aromatic polycarboxylic acids, such as terephthalic acid, are important chemical intermediates used for the production of industrially significant products, including polyester polymers, which can be used for fibre production and in the manufacture of containers, bottles and other molded articles.
Current technology for the manufacture of terephthalic acid involves the liquid phase oxidation of an aromatic feedstock, such as paraxylene, using molecular oxygen in a solvent. The oxidation solvent comprises a lower (e.g. C2-C6) aliphatic carboxylic acid, usually acetic acid and water, in the presence of a dissolved heavy metal catalyst system usually incorporating a promoter, such as bromine. Acetic acid is particularly useful as the solvent since it is relatively resistant to oxidation and increases the activity of the catalytic pathway for the oxidation of aromatic feedstock and reaction intermediates. The reaction is carried out in one or more vessels under elevated temperature and pressure, in the range of about 150 to 250° C. and 6 to 30 barA respectively and typically produces crude terephthalic acid (CTA) in high yield, e.g. at least 95%. Under these conditions the CTA precipitates from the solvent in the oxidation reactor(s) to form a slurry of CTA solids in oxidation solvent, which is maintained in suspension by agitation. The temperature of the slurry is reduced by passing through a series of crystallisers, each at successively lower pressure, before the CTA solids are separated from the oxidation reaction solvent to give the oxidation mother liquor. The separation of the CTA solids from the oxidation mother liquor occurs at positive pressure or under vacuum.
The liquid phase oxidation of paraxylene is an exothermic reaction and the solvent such as acetic acid and water are typically removed as an overhead vapor stream from the oxidation reactor as one means of controlling the temperature of reaction. The vapor stream is condensed to recover condensables, some of which may be recycled as reflux to the oxidation reactor, while other of the condensables are passed to a separation process which, in turn, allows for recovery of the aliphatic carboxylic acid solvent, having a reduced water content.